"The more secret information you know, the more successful the concealment of the plaintext."
It is important to realize that any crypto system in its design is an exercise in resource allocation and optimization.
If we were to return to the postal analogy used in the discussion of Asymmetric Ciphers. Suppose Alice has a secret message to send to Bob in the mail. Alice could put the message in her lock box and use Bob's padlock to lock it allowing Bob to open it with his key, as describe earlier. But if it were a really important message or Alice and Bob had a higher expectation of the opponent they wished to thwart (Bob's girlfriend knows where Bob keeps his keys) Alice and Bob might want to resort to a more complicated crypto system. For example Bob could have multiple keys, one he keeps on his key chain, one he keeps in a rented Post Office box and one that is in a box in a Swiss bank vault. Bob might welcome this sort of security for really serious messages but for day to day messages between Bob and Alice Bob will no doubt find a daily flight to Switzerland rather expensive inconvenient. All crypto systems must face a resource trade-off between convenience and security.